1. Field of the Invention
The present invention relates to an internal combustion engine control apparatus and more particularly to an internal combustion engine control apparatus provided with a knock detection function for detecting a knock caused in an internal combustion engine.
2. Description of the Related Art
To date, a method has been known in which a knock phenomenon caused in an internal combustion engine is detected by use of a vibration sensor. In this method, a knock is detected in such a way that the level of an inherent-frequency vibration in a predetermined knock detection window is extracted based on a fact that when the knock is caused during operation of an internal combustion engine, there is caused a vibration having an inherent frequency corresponding to the internal combustion engine or the vibration mode of the knock. It is commonly known that an inherent-frequency vibration level is extracted through a method utilizing an analogue band-pass filter circuit, or by performing digital-signal processing such as a short-time Fourier transformation (referred to as “SIFT”, hereinafter) or a discrete Fourier transformation (referred to as “DFT”, hereinafter).
Meanwhile, in the extracted inherent-frequency vibration level, there may be superimposed electric noise or mechanical noise, caused by seating of an intake valve or an exhaust valve or caused due to starting or ending of fuel injection by an injector. Accordingly, in the technology disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2006-348764), attention is focused on the fact that while a knock waveform formed of the level of a vibration caused by a knock gradually attenuates after reaching its peak, an electric-noise waveform and a mechanical-noise waveform formed of the vibration level of mechanical noise rapidly attenuate after reaching their peaks, and switching is performed between a first frequency and a second frequency, which are preliminarily set, in accordance with the operation status of an internal combustion engine, so that an inherent frequency, at which the knock waveform conspicuously appears, is obtained.
More specifically, in the technology disclosed in Patent document 1, the frequency is switched to the first frequency in a first operation status and is switched to the second frequency in a second operation status. Then, in the first frequency, determination on the occurrence of a knock is performed based on the peak value of a vibration level; in the second frequency, determination on the occurrence of a knock is performed based on the integration value of a vibration level.
In the conventional technology disclosed in Patent Document 1, the integration value of the level of a second-frequency vibration is utilized for the second frequency on which mechanical noise and electric noise are superimposed, and the area of a vibration level, in a knock detection window, that includes a knock waveform and the area of a vibration level, in the knock detection window, that includes a mechanical noise waveform are compared with each other; therefore, the S/N ratio can be raised in comparison with a case where the peak value of a vibration level, in a knock detection window, that includes a knock waveform and the peak value of a vibration level that includes a mechanical noise waveform are compared with each other.
Due to various factors such as environmental conditions (e.g., the temperature of oil in the internal combustion engine and the temperature of coolant water) in the combustion chamber of an internal combustion engine and variations in internal combustion engines (e.g., variations in the compression ratios in combustion chambers), the starting position of a knock waveform for a crank angle from the compression top dead center varies. Therefore, it is difficult to appropriately set a knock detection window in such a way that the respective edges thereof are aligned to the starting position and the end position of the knock waveform; thus, the area of the knock detection window needs to be set wider to some extent. In contrast to the conventional technology disclosed in Patent Document 1, in the technology disclosed in Patent Document 2 (Japanese Patent No. 3083546), vibration levels (that denote, in Patent Document 2, spectrum lines calculated through implementation of STFT) in a knock detection window are moving-averaged every predetermined spectra, and based on the moving-averaged spectrum lines, determination on the occurrence of a knock is performed.
In the conventional technology disclosed in Patent Document 2, even though the area of a knock detection window is set wider to some extent, moving-average processing is performed in a period (predetermined spectrum lines) corresponding to the width between the starting position and the end position of a knock waveform, so that a period is grasped in which the level of a vibration caused by a knock conspicuously appears; therefore, as is the case where the integration value of a vibration level, in the technology disclosed in Patent Document 1, is utilized, the vibration levels of mechanical noise and electric noise can be reduced.